Time-lapse seismic survey of a reservoir region

Abstract

A method of investigating a reservoir region in a subsurface formation by a time-lapse seismic survey, which subsurface formation comprises a further formation region adjacent to the reservoir region, which method comprises the steps of obtaining data from a time-lapse seismic survey including seismic data of the subsurface formation at a first point in time and a later point in time, processing the seismic data to obtain a seismic representation of change in a predetermined seismic parameter in the further formation region, which seismic parameter is dependent on stress; interpreting the seismic representation of change in the seismic parameter in the further formation region for an indication of changes of stress distribution in the further formation region; and deriving a property of the reservoir region using the indication of change of stress distribution in the further formation region.

Description

CLAIM TO PRIORITY [0001] This application claims priority based on U.S. Provisional Patent Application No. 60 / 513,955, filed Oct. 24, 2003.FIELD OF THE INVENTION [0002] The present invention relates to a method of investigating a reservoir region in a subsurface formation using a time-lapse seismic survey. BACKGROUND OF THE INVENTION [0003] Time-lapse seismic surveying is increasingly used for studying of earth formations. It is applied for monitoring of hydrocarbon bearing underground reservoirs, in particular to follow the effects resulting from producing reservoir fluids (e.g. oil, gas, water) through a well to surface. [0004] In time-lapse seismic surveying, seismic data are acquired at least two points in time. Time is therefore an additional parameter with regard to conventional seismic surveying. This allows studying the changes in seismic properties of the subsurface as a function of time due to, for example, spatial and temporal variation in fluid saturation, pressure and t...

AI Technical Summary

Benefits of technology

[0021] Further, the change in pore pressure in at least part of the reservoir region can be estimated, suitably using reservoir simulation. A geomechanical simulation can then calculate the effects of a change in pore pressure and pore-pressure distribution on the position of boundaries and the stress distribution in the subsurface formation, including the regions outside the reservoir region. In this way a geomechanical representation of changes in stress in the reservoir region and in the further formation region, and of displacements of region boundaries is obtained.

[0022] In a further step, the results of the geomechanical simulation can be used to predict the effects of displacements and of the change in stress on the predetermined seismic parameter, so that a geomechanical representation of the change in the seismic parameter is obtained.

[0023] The seismic and geomechanical representations of the changes in the seismic parameter can then be compared in order to interpret the time-lapse seismic survey. If there is good agreement between the seismic and geomechanical representations, then this is a validation of parameters used in the geomechanical model, and optionally reservoir model, in particular of the estimated pore pressure change. If there are differences, then an updated geomechanical representation can be determined, if needed in an iterative process, by changing parameters used in the geomechanical simulation and / or reservoir simulation of the subsurface formation. In particular, an improved estimate of the change in pore pressure in at least part of the reservoir region can be obtained. Thus, the method of the present invention can be used to investigate quantitative properties of the reservoir region. It will be understood, that also qualitative properties of the reservoir region can be derived, such as the sealing nature of a fault or discontinuity in the reservoir region, or the presence of areas in the reservoir that are undepleted or less depleted during production from the reservoir.

Problems solved by technology

A general difficulty in seismic surveying of oil or gas fields is that the reservoir region normally lies several hundreds of meters up to several thousands of meters below the earth's surface, but the thickness of the reservoir region or layer is comparatively small, i.e. typically only several meters or tens of meters.

Resolution of processed seismic data in the reservoir region is therefore an issue.

Whereas the resolution of the processed seismic data is often just sufficient to obtain a more or less clear indication of the presence of a discontinuity or fault, differences observed in the reservoir region in a time-lapse seismic survey (e.g. amplitude or interval velocity changes) are generally so small that it proves to be very difficult to draw reliable conclusions about the sealing nature of the fault.

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